It is known that vulcanizable rubber compositions comprising a nitrile group-containing highly saturated copolymer rubber having incorporated therein an organic peroxide and a zinc salt of an ethylenically unsaturated carboxylic acid give rubber materials having high strengths (For example, Japanese Unexamined Patent Publication [hereinafter abbreviated to "JP-A"] No. S63-270753 and JP-A H1-306443). These rubber materials are suitable for uses for which good weather resistance thermal resistance, strengths and oil resistance are required, such as various automobile transmission belts. However, they are often unsatisfactory in many other uses for which special characteristic performances are required.
It is also known that vulcanizable rubber compositions comprising rubbers, other than a nitrile group-containing highly saturated copolymer rubber, having incorporated therein an organic peroxide and a zinc salt of an ethylenically unsaturated carboxylic acid give rubber materials having high strengths. For example, JP-A S53-125139 discloses a rubber composition comprising poly-butadiene rubber having incorporated therein methacrylic acid, zinc oxide and an organic peroxide, and it is taught that a vulcanizate made therefrom is suitable for a solid golf ball. JP-A S53-85842 discloses a vulcanizate made from a composition comprising a diene rubber, methacrylic acid, zinc oxide, an organic peroxide and a non-polymerizable carboxylic acids which has a high tensile strength.
Rubber rolls used under a high load, such as, for example, steel-making rolls and paper-making rolls, have a problem such that a heat build-up occurs to a great extent when they are compressed or a shearing force is applied thereto. To solve this problem, it is considered to increase the hardness of rubber rolls. However, the increase of the rubber hardness results in deterioration of good rubber characteristics and occurrence at cracks on the surface of the rubber rolls.
The inventors have found that a vulcanizate made from a rubber having incorporated therein a zinc salt of an ethylenically unsaturated carboxylic acid exhibits a lower heat build-up than that of a vulcanizate made from a rubber having not incorporated therein the zinc carboxylate, if the two vulcanizates have a hardness of the same level. However, when a roll made of the rubber vulcanizate having incorporated therein a zinc salt of an ethylenically unsaturated carboxylic acid is used under severe conditions, it is not completely satisfactory.
It is known to use magnesium oxide as a vulcanizer for rubbers such as an acrylic rubber, a chloroprene rubber and a chlorinated polyethylene rubber, it is also known to incorporate zinc oxide in a rubber composition to enhance adhesion of the rubber composition to fiber or enhance thermal resistance of rubber parts made therefrom. As the magnesium oxide, soft-burned magnesia (burned at a relatively low temperature [i.e., not higher than 800.degree. C.]) is generally used.
JP-A H3-748 discloses a rubber composition comprising an acrylonitrile-butadiene copolymer rubber having incorporated therein an .alpha.,.beta.-unsaturated carboxylic acid, and an oxide, hydroxide or carbonate salt of zinc and/or magnesium, and teaches that a vulcanizate thereof has good oil resistance and hardness. JP-A H4-4240 discloses a rubber composition comprising an acrylonitrile-butadiene copolymer rubber having a high acrylonitrile content and having incorporated therein an .alpha.,.beta.-unsaturated carboxylic acid, an oxide, hydroxide or carbonate salt of zinc and/or magnesium, and an organic peroxide, and teaches thaw a vulcanizate thereof has good oil resistance and permanent set. It is noted, however, that these documents do not specifically disclose any working example wherein an oxide, hydroxide or carbonate salt of magnesium is used, and do not disclose any details of the magnesium oxide referred to.